T. muscularis of the gizzard of a chicken (HE)
Dorsally and ventrally, the T. muscularis forms a well-developed layer of smooth muscular tissue, the mm. crassi, that sometimes macroscopically gets mistaken for striated muscle tissue. Cranially and caudally, the gizzard forms two blind pockets with a lesser developed muscular layer, the mm. tenues. In the central part of the gizzard, the muscular layer is the thinnest and it is almost completely replaced by a tendinous layer of regular connective tissue, the centrum tendineum. The mm. crassi exists of three layers, a thin outer longitudinal layer, a massive circular layer and an internal (rather oblique) layer.